1. Field of the Invention
This invention relates to the structure of hook elements made of thermoplastic resin and molded integrally with a flat base of a surface fastener, and more particularly to the structure of molded hook elements having the softness and strength similar to those of monofilamentary hooks so as to improve an intermeshing force.
2. Description of the Related Art
There are currently known a variety of surface fasteners including hook elements which are made of woven and cut monofilaments. Such surface fasteners are characterized by the softness of fabrics and monofilaments, thereby assuring strong fastening with and smooth unfastening from a mating pile surface. Further, the monofilaments undergoing an elongation process have excellent tensile and bending strength even when the monofilaments are very thin. Monofilamentary hooks can be spaced very densely according to textile weave, thus causing very reliable fastening and being durable in repeated use.
Partly since materials for such woven surface fasteners are very expensive and partly since they are made through a number of manufacturing processes, it is very difficult to reduce the manufacturing cost.
To overcome the above inconveniences of the woven surface fasteners, integrally molded surface fasteners have been developed, in which hook elements integrally extend from a base by extrusion or injection molding.
Surface fasteners of the integrally molded type are disclosed in, for example, Japanese Patent Publication No. 22768/1973, Japanese Utility Model Laid-Open Publication No. 20509/1981, U.S. Pat. No. 3,312,583, Japanese Patent Laid-Open Publication No. 501775/1989.
According to Japanese Utility Model Laid-Open Publication No. 20509/1981, in production a flat base and straight projections extending from the flat base are molded integrally. Then tips of the straight projections are pressed against semi-spherical grooves on dies, being deformed according to the shape of the grooves, and being shaped as hook elements. In this case, the tips of the hook elements face random directions, thereby causing fastening force to be applied in arbitrary directions and providing very strong fastening. Partly since it is however very difficult to engage the straight projections with the grooves on the dies, and partly since the hook elements are not shaped in one process, not only the processes for making the hook elements become complicated but also respective hook elements are difficult to face desired directions and to be shaped as desired. In addition, the molded hook elements are not always uniform in shape, and are instable in intermeshing with loop elements on a companion surface fastener. Therefore the proposed surface fasteners have not yet been in practical use.
With the methods disclosed in Japanese Patent Publications No. 22768/1973 and No. 37414/1977, the base and the hook elements are integrally molded by extrusion, being shaped in succession. In production, die discs and spacers are superimposed one over another so as to obtain a mold drum. Then melted thermoplastic resin is extruded over the surface of the mold drum in rotation, being pressed into mold cavities on the die discs. The thermoplastic resin on the surface of the mold drum is compressed so as to form a base. The spacers are retracted inwardly so as to scrape a strip of surface fastener member from the surface of the mold drum. The die discs have a series of the mold cavities extending inwardly from outer side edges thereof with a predetermined spacing. The outer side edges of the spacers are smooth.
In the methods exemplified by U.S. Pat. No. 3,312,583 and Japanese Patent Laid-Open Publication No. 501775/1989, the base and the hook elements are formed integrally on a mold drum by extruding thermoplastic resin over a surface of the mold drum and into cavities of the mold drum in rotation. Then the base and hook elements are scraped from the surface of the mold drum according to the rotation of the mold drum. In these cases, the spacers are left in the mold drum. These methods and devices therefor are simplified, compared with the method of Japanese Patent Publication No. 22768/1973.
The spacers are necessary since one die disc cannot be formed with the cavities in complete conformity with the shape of the hook elements. Further, the tips of the cavities should face the circumferential of the die discs. Therefore the hook elements thus molded are directional.
To prevent the hook elements from being directional, integrally molded surface fasteners having hook elements with spherical tips is currently known.
As described above, hook elements in integrally molded surface fasteners cannot be shaped as delicately as those of woven surface fasteners. Molded hook elements do not have sufficient molecule orientation. Therefore when it is as thin as a monofilament, each hook element is too weak to be applied for practical use. The hook element should be inevitably large so as to be strong enough. In such a case, the hook elements become not only too rigid but also the number of the hook elements per unit area (density of the hooks) is reduced, resulting in weaker intermesh with loop elements of the companion fastener member. To compensate for this inconvenience, when the tips of the hook elements are made rigid, fastening and unfastening become somewhat unsmooth and stiff. In addition, the loop elements on the companion surface fastener member may be damaged, cut or worn out easily. Therefore, surface fasteners of this type are used only for installing interior finish materials for which repeated fastening and unfastening is not carried out frequently, for example.
This tendency is remarkable in the hook structure disclosed in Japanese Patent Publication No. 22768/1973. Specifically, the tips of hook elements have a cross-sectional shape of reverse triangle so as to facilitate stripping of molded surface fastener members. Therefore, the apexes of the reverse triangular tips of the hook elements intermesh with the loop elements of the companion surface fastener member, thereby causing the loop elements to be broken easily. The hook element disclosed in U.S. Pat. No. 3,312,583 is not necessarily required to be triangular in cross section, thereby reducing a possibility of breakage of the loop elements.
The integrally molded hook elements described above are very simple in cross section at their portions extending from the base, being easy to bend. When they are used repeatedly, the hook elements cannot restore their original posture, failing to intermesh with the loop elements.
Japanese Utility Model Laid-Open Publication No. 20509/1981 discloses that the tips of the hook elements are circular in cross section and are arcuate so as to assure smooth fastening and unfastening. Since they extend perpendicularly from a base, the tips of the hook elements are not only difficult to grip the loop elements but also are easily deformed at the portions where the tips of the hook elements are curved when unfastening from the loop elements. Although the hook elements are not broken, they remain deformed, thereby making the products less valuable.